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The α-β phase transition in volcanic cristobalite

Damby, D.E.; Llewellin, E.W.; Horwell, C.J.; Williamson, B.J.; Najorka, J.; Cressey, G.; Carpenter, M.A.

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Authors

D.E. Damby

B.J. Williamson

J. Najorka

G. Cressey

M.A. Carpenter



Abstract

Cristobalite is a common mineral in volcanic ash produced from dome-forming eruptions. Assessment of the respiratory hazard posed by volcanic ash requires understanding the nature of the cristobalite it contains. Volcanic cristobalite contains coupled substitutions of Al3+ and Na+ for Si4+; similar co-substitutions in synthetic cristobalite are known to modify the crystal structure, affecting the stability of the and forms and the observed transition between them. Here, for the first time, the dynamics and energy changes associated with the – phase transition in volcanic cristobalite are investigated using X-ray powder diffraction with simultaneous in situ heating and differential scanning calorimetry. At ambient temperature, volcanic cristobalite exists in the form and has a larger cell volume than synthetic -cristobalite; as a result, its diffraction pattern sits between ICDD - and -cristobalite library patterns, which could cause ambiguity in phase identification. On heating from ambient temperature, volcanic cristobalite exhibits a lower degree of thermal expansion than synthetic cristobalite, and it also has a lower – transition temperature (473 K) compared with synthetic cristobalite (upwards of 543 K); these observations are discussed in relation to the presence of Al3+ and Na+ defects. The transition shows a stable and reproducible hysteresis loop with and phases coexisting through the transition, suggesting that discrete crystals in the sample have different transition temperatures.

Citation

Damby, D., Llewellin, E., Horwell, C., Williamson, B., Najorka, J., Cressey, G., & Carpenter, M. (2014). The α-β phase transition in volcanic cristobalite. Journal of Applied Crystallography, 47(4), 1205-1215. https://doi.org/10.1107/s160057671401070x

Journal Article Type Article
Acceptance Date May 9, 2014
Online Publication Date Jun 14, 2014
Publication Date Aug 1, 2014
Deposit Date May 30, 2014
Publicly Available Date Jun 17, 2014
Journal Journal of Applied Crystallography
Print ISSN 0021-8898
Electronic ISSN 1600-5767
Publisher International Union of Crystallography
Peer Reviewed Peer Reviewed
Volume 47
Issue 4
Pages 1205-1215
DOI https://doi.org/10.1107/s160057671401070x
Keywords Cristobalite, Phase transitions.

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Publisher Licence URL
http://creativecommons.org/licenses/by/4.0/

Copyright Statement
Advance online version This is an open-access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.





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